1. Field of the Invention
This invention relates to the use of selected extractive distillation agents to separate tetrafluoroethylene (TFE) and hydrogen chloride (HCl).
2. Background
Tetrafluoroethylene can be made by the dehydrochlorination o Generally, it is made by the pyrolysis of chlorodifluoromethane (CF.sub.2 HCl). Ideally, two moles of CF.sub.2 HCl form one mole of TFE and two moles of HCl. Sometimes steam is used as a diluent in the pyrolysis, but this leads to problems, such as equipment corrosion and the formation of undesirable by-products (see U.S. Pat. No. 4,849,554). The HCl recovered is contaminated with water and high-boiling fluorocarbons. Aqueous HCl is of considerably less value than anhydrous HCl, which can be used in various chemical reactions.
Another process disclosed in the aforesaid patent uses CO.sub.2 as a diluent, but experiments have which causes contamination of the waste HCl with HF when the HCl is separated from the TFE by scrubbing with water.
Other known processes for the pyrolysis of CF.sub.2 HCl are also anhydrous, and may use recycle gases as diluents. In such processes, TFE is separated from unconverted reactant and from by-products by distillation, and is recovered as its HCl azeotrope. This is desirable in that the azeotrope is much less explosive than pure TFE, and can be stored and handled with greater safety. Before use for polymerization, however, it is necessary to separate the TFE from HCl, and this is usually done by scrubbing with water to make dilute waste aqueous HCl, then with caustic to remove traces of HCl, and then with cold glycol to remove most of the water. The monomer is then dried, inhibited, compressed, stripped to remove low boilers, and finally refined to remove inhibitor and other high boilers. This is a complex process, requiring extensive equipment, and produces aqueous HCl of little value as a by-product.
It would be desirable to find a way to separate TFE from HCl in the CF.sub.2 HCl pyrolysis crude product without diluting the HCl with water.
At normal processing conditions of pressure and temperature, the azeotrope composition is approximately 67 mole percent HCl and 33 mole percent TFE. The composition of the azeotrope changes very little with temperature and pressure. Therefore, breaking the azeotrope by operating at extremely low or high pressure cannot be accomplished.